States of water in partially swollen poly(vinyl alcohol) hydrogels

A poly(vinyl alcohol) hydrogel was prepared by Coupling poly(vinyl alcohol) with epichlorohydrin as cross-linking agent. Various amounts of water were added into the dry hydrogel to swell it and the quantity of water in various states in the partially swollen hydrogel were determined by DSC ice-melting technique. A parameter S denoted the degree of the saturation of swelling was introduced for describing the various stages of water absorption. The analytical results indicate that there exists two critical threshold degree of saturation. At the first threshold S*, the freezable bound water starts to appear and at the second threshold S**, the free water begins to appear. In the region of S < S*, the adsorbed water exists only in the non-freezable form and increases linearly with S until reaching S*. The sum of non-freezable and freezable bound water consists of the hydration layer around the chain segments of the network. The hydration number HN of the repeating unit of PVA hydrogel increases from zero to a limiting value about 2.5 as the degree of saturation varies from zero to one. (c) 2005 Elsevier Ltd. All rights reserved.